In conventional rotary drilling or earth boring operations, stabilization tools are placed above the drill bit in the drilling string. These stabilization tools employ two basic methods of maintaining the orientation of the drill bit about its axis and ideally, also the drill string axis of rotation. Such tools minimize drift of the bore hole from the vertical or any other preferred azimuthal angle. The first method is tool rigidity itself and the second method is tool contact with the well bore wall. Inasmuch as continual circulation of drilling fluid down through the inner bore of the drill string and returning up through the annular area between the drill string and the bore hole wall must be maintained, the second method of stabilization is most usually obtained through the use of "ribs", "ridges" or "blades" which protrude out from the main body of the tool in contact with the bore hole wall. The interstitial area between these blades provides the annular area or volume necessary for return of circulating fluid used in rotary drilling operations.
One example of the present invention occurs in the tool commonly known as a square drill collar which combines the utilization of both methods and an example of such tool can found in U.S. Pat. No. 3,343,615. The body of the tool is heavy walled and rigid, while at 90.degree. intervals around the circumference of the tool, ribs protrude to make contact with the well bore and run the axial length of the tool, usually some 30 feet. The diagonal measure between opposite rib surfaces is very nearly exact hole size and the rib surfaces are rounded to effect maximum contact area with the bore hole wall.
Because the tool's rib surfaces are in constant contact with the bore hole wall to provide maximum stabilization and prevent azimuthal deviation, these ribs are provided with protection against the erosion and abrasion effected by hard abrasive geologic formations. If not protected by hard metal stripping or insertion of ultra-hard material into the mild carbon steel, the contacting surface will abrade and the tool will progressively lose its effectiveness. Use of such inserts in such a tool is disclosed for example in Canadian Pat. No. 658,049.
In the current technology, it is common to insert buttons of an ultra-hard material such as tungsten carbide into the mild steel of the tool body. Additional examples of such tools are found in U.S. Pat. Nos. 3,680,647; 4,060,286; and 2,288,124. These inserts must bear against the well bore wall, providing for tool contact, however, this ability to withstand abrasion makes the tool fitted with these inserts extremely difficult to free when wedged or stuck solidly downhole. When a drill string is "wedged" or "stuck" downhole, a number of means may be employed to release or "unstick" the drill string. Commonly a washover procedure is used as mentioned in U.S. Pat. No. 3,318,398.
In washing over, a large bit or "shoe" is used to pass over the outside wall of the drill string and mill away all obstructions between the nominal diameter of the drill string and the actual diameter of the well bore. When this washover shoe encounters aluminum, rubber, iron or mild steel, it can effectively mill away the materials. If, however, the conventional shoe encounters ultra-hard material such as tungsten carbide buttons used in the stabilizing tools, it is woefully ineffective and is often broken or severly damaged, bringing washover operations to a halt and leaving the drill string still "wedged" in the bore hole. It is possible to use a diamond tooth washover shoe to mill away the hard metal obstructions. However, these shoes must be custom made and the expense and fabrication and drilling downtime is often so high as to make this course of action less desirable than abandonment of the well.